In the following background discussion as well as in the disclosure of the present invention, the following abbreviations will be frequently used:
BL brush-less
DC direct current
DCV directional control valve
EH MUX electro-hydraulic multiplexer
ESD emergency shut down
I/O input/output
LVDT linear variable differential transformer
PM permanent magnet
PSD production shut down
SIL safety integrity level
SHPU sub sea hydraulic power unit
SMA shape memory alloy
XMT, Xmas tree Christmas tree
The prior art in control systems for hydrocarbon production comprises both hydraulic and electrical control, respectively.
Most concepts for electrical actuation of large gate valves include the use of an electrical motor and a roller screw or other form of rotary-to-linear mechanical conversion device, such as disclosed in e.g. U.S. Pat. No. 7,172,169 and in U.S. Pat. No. 6,572,076. Other concepts, such as disclosed in NO 322680, are based on use of a small SHPU, to combine the action of an electrical motor and a hydraulic piston/cylinder arrangement. Both approaches have merits and both also have certain limitations. The former approach tends to involve mechanical complexity and extensive instrumentation in non-retrievable components (i.e. for example integrated with an XMT module) and large dimensions. The latter approach tends to involve several hydraulic components demonstrated over many years to have less than desirable reliability in a sub sea context, e.g. DCV pilot valves requiring high fluid cleanliness for reliable operation, pressure relief valves and hydraulic accumulators. The latter, if in the form of nitrogen (N2) charged bladder design, are prone to leakage over time, which is the reason they are normally carried on easily retrievable modules. In deep water N2 charged accumulators are also inefficient. In the form of mechanical spring charged designs accumulators are bulky and unsuitable to be part of an actuator located on e.g. an XMT.
The present invention is based on a combination of principles pursued in both camps (roller screw and hydraulics) as per the above, and especially on the use only of the best components from each camp in a combination exhibiting unparalleled robustness and reliability combined with cost effectiveness.
The critical feature of a sub sea valve actuator as applied to e.g. an XMT is in the fail safe latch arrangement. This is a mechanism designed to work in conjunction with a return spring, the latter storing energy required to turn the valve from the production position to the safer position, usually from open to closed position.
For the case of electromechanical operation the latch is usually also electromechanical. Many versions have been devised, but few implemented and commissioned in the sub sea industry.
For the local (to the actuator) SHPU line of approach the fail safe feature is almost invariably provided by means of a DCV. Such valves have several unfortunate, but necessary design features. Traditionally the DCV has not been critical to the ESD functionality, except for a few installations characterised by very long offset of the sub sea production facility from the host platform. The universally accepted form of ESD for a traditional EH MUX production control system is in the form of hydraulic bleed down from the host platform, thus the safety critical DCVs are located on the host platform, and are thus accessible for repair or replacement.
Use of pressure relief valves sub sea has very little, if any, history in production control systems. The industry has shunned pressure regulating valves and pressure relief valves used sub sea. The full range of valves normally used in a mini SHPU dedicated to control of a single actuator are basically considered sensitive to particulate contamination and thus undesirable.
Electrical actuation should be defined in a system context, i.e. an actuator with only electrical (and possibly optical) interfaces, and no hydraulic interfaces, to the upstream parts of the production control system. FIG. 1 illustrates a typical prior art SHPU circuit pursued by several designers for achievement of an actuator using hydraulic components. The concept includes a pump driven by an electric motor, an accumulator for storage of hydraulic power, usually a filter for cleaning the fluid, and a solenoid operated DCV for directional control and a cylinder/piston unit. The latter is interfaced to the valve stem, providing the forces to bring the valve to the production position. A large return spring is usually provided for storage of the energy required to return the valve to the safe position when the hydraulic pressure is vented by the DCV when the solenoid is de-energized.
With reference to FIG. 1, it is customary to organise a motor 1 connected to a pump 3 via a flexible coupling 2 to generate a pressure and a flow through check valve 13 such as to charge an accumulator 8. A pressure relief valve 5 is arranged as indicated in FIG. 1 for protection of the pump and motor. Upon actuation, pilot valve 10 drives DCV 9 to the operating position to let fluid through connector 19 to actuator cylinder 11 and to drive a piston in cylinder 11 to the open position of the valve 12, also pushing fluid out from the spring side of the cylinder 11 through connector 20 and check valve 17 and filter 15. When the valve is to be returned to the safe position the solenoid of the pilot valve 10 is de-energized, the DCV 9 is driven under the force of the spring to vent the pressure in the cylinder 11 and the spring side of the piston sucks fluid from reservoir 7 through hydraulic connector 20 and check valve 18. The absolute pressure in the circuit is high for deep water and minor pressure drops across filters is of no consequence.
This circuit is suitable for a topside installation where the components most sensitive to contamination, notably DCV pilot valve 10 and pressure relief valve 5 may be accessed for repair or replacement, and where the ambient pressure at 1 bar is suitable for use of a nitrogen charged accumulator 8, but less suitable for a sub sea installation.
The present invention aims for elimination of these three undesirable components, but still providing an operable actuation system of great robustness and reliability.
In the following, several features of radical improvement on this concept with respect to reliability in operation will be described as parts of the present invention.